The present invention relates to an apparatus for compensating for the irregularities of illumination for a copier of the type having a lens which is selectively movable along and perpendicularly to an optical axis thereof.
In a slit exposure type copier, when a lens is moved along or perpendicularly to an optical axis thereof for changing magnification and shifting an image, the illumination distribution on a photoconductive element is varied in various manners on account of cos fourth-power rule, as well known in the art. Generally, in a copier of the type described, magnification is changed by moving a lens in different directions depending upon the reference position for laying an original document, i.e., by moving it along its optical axis if the reference position is the center and in an oblique direction if the reference position is the end. Heretofore, the amount of shift of the lens, whether it be parallel to or perpendicular to the optical axis, has been determined unconditionally in correspondence to magnification, and irregularities of illumination on a photoconductive element have been compensated for by picking up the displacement along or perpendicular to the optical axis by means of a cam so as to move a shield plate. However, in the slit exposure type copier wherein the lens is movable in the two directions as desired to implement image shift and magnifications change as described, the prior art approach which unconditionally moves a shield plate for a particular magnification cannot fully compensate for irregularities of illumination on a photoconductive element.